spi: Avoid using malloc() in a critical function

spi: Avoid using malloc() in a critical function

In general we should avoid calling malloc() and free() repeatedly in
U-Boot lest we turn it into tianocore. In SPL this can make SPI flash
unusable since free() is often a nop and allocation space is limited.

In any case, these seems no need for malloc() since the number of bytes
is very small, perhaps less than 8.

Signed-off-by: Simon Glass <sjg@chromium.org>
Fixes: d13f5b254a (spi: Extend the core to ease integration of SPI
	memory controllers)